This invention relates to an improved DC semiconductor electric motor of an axial-air-gap type.
Semiconductor motors of various types of construction have been proposed. For musical instruments, for example, motors of the axial-air-gap type are preferred because this type can be made sufficiently flat in shape. Semiconductor axial-air-gap type motors also have the additional advantage of higher efficiency, better stability in performance, and improved durability. Notwithstanding these advantages, however, axial-air-gap type motors sometimes have had several difficulties. First, in a semiconductor axial-air-gap type motor, the field magnet usually serves as the rotor, resulting in large frictional losses caused by a large thrust load on the rotating shaft of the rotor due to axially-oriented magnetic forces on the shaft. While ball bearings, for example, may be used to reduce such frictional losses, their cost, however, will increase the ultimate cost of the motor and renders their use uneconomical.
Second, lap-wound or wave-wound armature coils are usually employed for achieving a high efficiency, and are arranged in a superimposed manner. Such a superimposed arrangement of the coils, however, increases the amount of air-gap in the magnetic field, thus resulting in a decrease in the magnetic field strength, thereby decreasing the efficiency and the starting torque of the motor. The effect of such decreases in efficiency and torque will be especially larger in small-sized motors; accordingly, motors as small as about 40mm or less in diameter will be of little practical use.
Third, an arrangement of superimposed armature coils such as the above will require a large amount of space in the motor.
Motors of the salient-pole type are known to the art and contain superimposed armature coils and induction coils which are superimposed thereon in the motor casing. These induction coils produce a direct output current proportional to the rotating speed of the rotor, which output current may be used as a constant-speed control thereof. In an axial-air-gap motor, however, the use of induction coils, which are superimposed on the superimposed armature coils, is impractical and placement of the induction coils between the armature coils is not a feasible alternative because of the superimposed arrangement of the armature coils. Accordingly, it is difficult to obtain a direct output current proportional to the rotating speed of the rotor for use as a constant-speed control for the motor. On the other hand, other alternatives to induction coils are excessively costly.
Fourth, fabrication of lap-wound or wave-wound coils is complicated and not readily susceptible to mass production techniques.
Fifth, employment of a three-pole type motor for the purpose of simplifying construction will cause an unbalanced force acting perpendicularly on the rotating shaft resulting in a decrease in durability due to the resultant wear of bearings and in an increase in mechanical noise.
Accordingly, it is a general object of the invention to provide a semiconductor electric motor of an axial-air-gap type which is simple in construction, of high durability, less noisy, susceptible to mass production, and of low cost.
It is another object of the invention to provide an axial-air-gap type semiconductor motor wherein the magnetic field contains only a small amount of air-gap to effect a high efficiency.
It is a further object of the invention to provide an axial-air-gap type semiconductor motor which has a sufficiently large output power while the motor is small in size.
A still further object of the invention is to provide an axial-air-gap type semiconductor motor which is provided with a disc-shaped rotor constituting a field magnet, armature disc confronting respectively the upper and lower faces of the rotor, and armature coils affixed on the faces of the armature discs.
A still further object of the invention is to provide an axial-air-gap type semiconductor motor which is provided with a control band which is coaxial to and rotatable synchronously with a rotor, and position detectors which are located in positions along the control band to generate an output proportional to the rotating speed of the rotor, thereby to effect a constant-speed control for the rotor by means of an output from the position detectors through a current control circuit.
A still further object of the invention is to provide an axial-air-gap type semiconductor motor which is provided with induction coils which are located respectively at the central portions of the armature coils to generate an output proportional to the rotating speed of the rotor thereby to serve as a constant-speed control for the rotor.
The above and other objects and advantages of the invention will be apparent from the following description of preferred embodiments with reference to the drawings.